As long as none were sold before he admitted they were knock-offs no harm was done to require action I suppose.
However it would be worth noting for future reference that he did leave that fact out until asked (although he complied and admired it rather quickly). It's probably not worthy of earning a spot in the blacklist thread though (should there be a brownlist? Maybe another color? lol).
Just know that you're buying a knockoff, compare it to other headphones in the price range, and decide if you think they're worth the current asking price I suppose.